New Jersey at the time was a thinly populated agrarian state that actually had quite a lot more in common with the Upper South because of its foodstuff sales and cotton imports for the few textile mills, than with the burgeoning industrial centers in Pennsylvania and New York (neither of which had spilled over yet in 20th Century urban sprawl). People who lived in the state were conservatives through and through, and their descendants still are (more on this later).
At the time of the Civil War, New Jersey had a population of
several hundred slaves that wouldn’t be freed until the 13th Amendment, the state legislature passed a resolution expressing supportp for the cause of South Carolina, it was the only “free” state to vote against Abraham Lincoln both times, and was the last Northern state to ratify the 13th Amendment. Admittedly not that stellar, and only made up by the close to 90,000 New Jerseyans who served in the Union forces.
So what happened to turn the state into a liberal one? Simply put, New York City and Philadelphia exploded in size. In the northern half of the state, the sprawl spilled completely over the Hudson as business owners of New York’s chief industries, shipping, chemicals, furniture, etc, decided that the land was cheaper and the workers plentiful on the other side. In the South, the Delaware proved to be a much more effective barrier as all the raw materials for Philadelphia’s traditional industries (iron and steel, petrochemicals, war materiel) are on the Pennsylvania side, so spillover didn’t really happen until after the Pennsylvania Railroad put the first bridge across the river below Trenton in 1896, and was far more restrained than in the North.
These tendencies were further exacerbated by the Great White Flight out of both cities in the late 1950’s and 60which saw hundreds of thousands of middle class Whites move into the new suburbs created in what had been farmland a year prior. My Father was born in 1960 just after his family left Philadelphia, and has told me repeatedly about how our home town went from being literally a one traffic light town with just about 4,000 people living in farms and hamlets all over the place to 35,000 less than 30 years later.
Today, if you want to see what these demographic changes look like, take a look at a map and compare it to this map (
https://www.nj.com/news/index.ssf/2016/11/how_your_town_voted_in_the_2016_presidential_election.html) of the 2016 Presidential Election results by town. As can be quickly seen, the blue for Hillary roughly forms a half circle in the South with a radius on Philadelphia, plus stronger than usual support along a line to Atlantic City almost perfectly corresponding to the Atlantic City Expressway. In the North, it’s a line stretching along I-95 between Trenton and New York City. Almost everything else is red. That’s what happened to those conservative New Jerseyans. There all still there in their >5,000 people towns that make up most of the state’s land area, but they just got outnumbered by some big colonies of the two major cities.